The only way I've been able to route pfsense to an exit node is to first create an interface bound to the tailscale service, add the tailscale IP address tied to your device as static, and add the exit node you want in the upstream gateway field. Then, head over to the System->Routing->Gateways settings and edit the new gateway. Disable gateway monitoring and gateway monitoring action. Lastly, go into your firewall rules for your LAN that you want going into the tailscale vpn and set the gateway for each rule to the new gateway. Your devices should be routing to the exit node now.
This is not ideal, as your device IP could change at some point, but it's the only thing I got to work. I even tried pushing 0.0.0.0/1 and 128.0.0.0/1 as a subnet from the exit node to override the default route, and that worked at first, but as soon as the tailscale service itself needs to talk, it sends traffic within its own VPN and things fall apart.